It is known that the presence of an admixture changes various properties of a carrying fluid, such as density, color, radioactivity, magnetic and thermal properties and electrical resistivity. So, measurement of physical properties allows estimating a concentration of an admixture; in particular, compositions of a saline solution, a water-oil emulsion and other mixtures may be determined by measuring the electrical resistivity. The following concentration determining methods are known: visual (by admixture color), indirect (by flow conductivity), etc., (see, for example, M. Levy, B. Berkowitz, Measurement and Analysis of Non-Fickian Dispersion in Heterogeneous Porous Media, Journal of Contaminant Hydrology//2003, 64, pp. 203-226; Gas B, Zuska J, Coufal P, van de Goor T. Optimization of the High-Frequency Contactless Conductivity Detector for Capillary Electrophoresis, Electrophoresis//2002, v. 23, pp. 3520-3527).
The main problem of the known method is the averaged nature of measurements, i.e., a significant time interval determined by dimensions of a measurement cell within which an admixture concentration may change significantly. The disclosed method provides improved accuracy of determining the concentration of an admixture without changing configuration of a measurement cell.
The disclosed method for determining local changes of concentration of an admixture in a fluid flow comprises injecting the fluid through a measurement cell; the fluid contains an admixture a change of concentration of which in time at an inlet to the measurement cell is known. The admixture concentration change in time is determined in the measurement cell, then, an impulse response of the measurement cell is recovered applying a method of deconvolution. A fluid being studied is injected through the measurement cell and a change of the admixture concentration in time in the fluid flow is determined in the measurement cell. The change of the admixture concentration in time in the flow of the fluid being studied at the inlet to the cell is found with the following equation:
            ∫      0      t        ⁢                  K        ⁡                  (                      t            -            τ                    )                    ⁢              I        ⁡                  (          τ          )                    ⁢                          ⁢              ⅆ        τ              =            R      σ        ⁡          (      t      )      where τ—an integration variable, t—time, I(t)—change of the admixture concentration in the flow of the fluid being studied at the inlet to the cell, Rσ(t)—change of the admixture concentration in the flow of the fluid being studied in the measurement cell, K(t)—the impulse response of the measurement cell.
A dependence of a physical property of the fluid on a concentration of the admixture may be determined preliminarily; in this case, the change of the admixture concentration in the fluid flow in the measurement cell is determined by measuring the physical property of the fluid.
The fluid property being measured is an electrical resistivity, a density, a radioactivity, etc.
A quality of the measurement cell may also be estimated; for this purpose, a difference between the concentration measured in the measurement cell and the concentration at the inlet to the measurement cell is determined and the quality of the measurement cell is estimated based on the determined difference.
The quality of the measurement cell may also be estimated by determining the impulse response of the measurement cell by the Fourier transformation and comparing the Fourier transformation from the function K(t) with the constant 1/√{square root over (2π)}.